Effects of β-agonist zilpaterol hydrochloride feeding and supplementation period on growth and carcass characteristics of Lori-Bakhtiari lambs

Fifty six Lori-Bakhtiari feedlot lambs (initial BW = 44 ± 4.7 kg) were used in a 2 × 3 + 1 factorial experimental design to evaluate the effects of 3 feeding regimens of zilpaterol hydrochloride (ZH) for the last 28 and 42 dbefore slaughter on growth performance, carcass characteristics and some plasma metabolites. The continuous feeding of ZH (daily regimen), intermittent 1 d feeding ZH followed by 1 d of withdrawal (1 on 1 off regimen), and intermittent 2 d feeding ZH followed by 2 d of withdrawal (2 on 2 off regimen) were employed as the different regimens. The lambs were assigned to one of seven treatments (8 lambs/treatment) based on initial BW. Lambs were fed a diet formulated to contain 14% CP and 2.45 Mcal/kg DM of metabolizable energy (ME) supplemented with 0.2 mg ZH/kg of live weight d−1. The basal diet without ZH was the added control group. Lambs fed ZH continuously for 28 d had higher (P < 0.001) feed efficiency compared to the control group and other intermittent regimens. Neither feeding regimen nor period had any significant effect (P > 0.05) on final BW, total weight gain, average daily gain and dry mater intake. Hot and cold carcass weights and dressing percentage were greater for daily feeding for either 28 or 42 d and for the 1 on 1 off-42 d treatments than for the control and the other groups fed ZH (P < 0.05). Longissimus dorsi muscle area was increased (P < 0.01) by daily-28 d compared to the control. Back fat thickness tended to be lower (P = 0.063) in pooled ZH-treated groups whereas there was no effect of ZH on the proportion of final weight of the fat-tail. Daily-28 d and 1 on 1 off-42 d groups had lower intramuscular fat content (P < 0.01) than the control group. Plasma concentrations of urea were decreased (P < 0.05) in daily-28/42 d groups compared to the control. Also, there were lower concentrations of urea after 42 d of ZH feeding compared to 28 d of ZH feeding (P < 0. 05). Overall, feeding ZH in daily for either 28 or 42 d before slaughter as well as for 42 d on alternate days improved feed efficiency ratio and some carcass characteristics. While backfat tended to be decreased by ZH there was no effect on fat-tail in Lori-Bakhtiari lambs.